Integrated circuits are made by forming on a substrate, such as a silicon wafer, layers of conductive material that are separated by layers of a dielectric material. Openings called vias and trenches may be etched through the dielectric layers, then filled with a conducting material to electrically connect the separated conductive layers.
A commonly used material to form a dielectric layer is silicon dioxide. Although a thermally stable and mechanically strong material, silicon dioxide has a relatively high dielectric constant. Consequently, certain materials such as various organic polymers that have a relatively low dielectric constant may be used as a dielectric material in place of silicon dioxide. When such materials are used in place of those with a higher dielectric constant, RC delay may be reduced, which can enable a higher speed device.
Etching is a process of removing selected portions of a layer from a wafer surface through openings in a hard mask with a specified resist pattern. Dry etching typically is used to obtain sufficient control and precision for integrated circuits with features below 3 μm. Dry etch techniques include plasma etching, ion beam etching, and reactive ion etching. Plasma etching requires a chemical etchant and an energy source. For example, a plasma etcher may include a chamber, vacuum system, gas supply, and power supply. The wafers may be loaded into the chamber, the pressure inside is reduced by the vacuum system, and the chamber is filled with the reactive gas. For example, to etch silicon dioxide, the gas may be CF4 mixed with oxygen. The energy source, such as a power supply that creates a radio frequency field through electrodes in the chamber, energizes the gas mixture to a plasma state. In the energized state, the etchant attacks the dielectric material, converting it into volatile components that are removed by the vacuum system.
Polymer-based dielectrics may be etched with chemical etchants that include and/or are based on O2-based chemistry. The etchant also may include additives such as N2, H2, or CO. Ion bombardment also may be used to etch a polymer dielectric material, in conjunction with a chemical etchant. Typically, some over-etching is done of the polymeric dielectric material. Over-etching may be needed to ensure complete removal of the selected material, at least in part due to variations in material thickness and etch non-uniformity across the surface.
However, over-etching also can cause or increase a problem referred to as undercutting. Undercutting is the unwanted removal of dielectric material below the edges of a mask. In general, etching of a dielectric can result in undercutting because etching tends to be somewhat non-directional, or isotropic, especially with polymer dielectrics. Undercutting results in sidewall surfaces that are not vertical, but are bowed. If etch time is increased, the etching may remove even more of the polymeric dielectric material from underneath the mask, worsening the undercutting problem. Although attempts to reduce undercutting have been made by simply reducing the over-etch time, this is not a practical solution because some over-etch is needed to compensate for material thickness variations and etch non-uniformity.
The undercutting problem is an obstacle to the development of smaller and faster devices, because undercutting can result in variations and departures from designs that seek to minimize spacing of dielectric materials between conductive elements such as conductive elements in vias and trenches, misalignment of conductors and/or insulating elements extending through or into the etched dielectric layers, and other similar problems. What is needed is a device without the undercutting problem, that has more vertical sidewalls on the sides of an opening in a polymer dielectric layer. A method for etching a polymer dielectric with a low dielectric constant is needed that will reduce, minimize or eliminate the undercutting problem.